Flyback converter with no-load power control for reduced power consumption

Abstract

A flyback converter is provided with a secondary-side low-power-mode controller that detects whether a mobile device has been disconnected from the flyback converter. In response to this detection, the low-power-mode controller initiates a low-power mode of operation in which a primary-side controller is disabled to increase efficiency.

We claim: 
     
       1. A flyback converter, comprising:
 a synchronous rectifier switch coupled to a secondary winding of a transformer; 
 a power switch; 
 a synchronous rectifier controller configured to control a cycling of the synchronous rectifier switch; and 
 a low-power-mode controller configured to detect a no-load condition for the flyback converter, wherein the low-power-mode controller is further configured to switch off the power switch to shut down the synchronous rectifier controller responsive to a detection of the no-load condition for the flyback converter and to switch on the power switch to power the synchronous rectifier controller responsive to a detection of a load condition for the flyback converter. 
 
     
     
       2. The flyback converter of  claim 1 , wherein the low-power-mode controller is further configured to monitor a voltage across an output sense resistor to detect the no-load condition. 
     
     
       3. The flyback converter of  claim 2 , wherein the low-power-mode controller is further configured to detect the no-load condition responsive to the voltage across the output sense resistor dropping below a disconnection threshold voltage. 
     
     
       4. The flyback converter of  claim 1 , wherein the low-power controller is further configured to monitor a data terminal for a data and charging cable to detect the no-load condition. 
     
     
       5. The flyback converter of  claim 4 , wherein the low-power controller is further configured to monitor the data terminal to determine whether a mobile device is connected to the flyback converter through the data and charging cable, and wherein the low-power controller is further configured to detect the no-load condition responsive to a detection that the mobile device is disconnected from the data and charging cable. 
     
     
       6. The flyback converter of  claim 4 , wherein the data and charging cable is a USB cable. 
     
     
       7. The flyback converter of  claim 1 , wherein the low-power-mode controller is further configured to determine whether an output voltage for the flyback converter has dropped below a first threshold voltage during a low-power mode of operation, and wherein the low-power-mode controller is further configure to terminate a low-power-mode of operation responsive to a detection that the output voltage for the flyback converter has dropped below the first threshold voltage. 
     
     
       8. A method for a flyback converter, comprising:
 during an absence of a no-load condition for the flyback converter, cycling a synchronous rectifier switch transistor, wherein a synchronous rectifier controller controls the cycling of the synchronous rectifier switch transistor; 
 detecting the no-load condition for the flyback converter; and 
 opening a switch coupled between a power supply voltage node and the synchronous rectifier controller in response to a detection of the no-load condition for the flyback converter to shut down the synchronous rectifier controller. 
 
     
     
       9. The method of  claim 8 , further comprising:
 detecting a disconnection of a mobile device from the flyback converter to detect the no-load condition.